King Bhumibol Adulyadej is followed by Queen Sirikit (second from left) and Princess Maha Chakri Sirindhorn (left) as he leaves a hospital in 2011.

Credit
Damir Sagolj/Reuters

King Bhumibol Adulyadej of Thailand, who took the throne of the kingdom once known as Siam shortly after World War II and held it for more than 70 years, establishing himself as a revered personification of Thai nationhood, died on Thursday in Bangkok. He was 88 and one of the longest-reigning monarchs in history.

The royal palace said he died at Siriraj Hospital but gave no further details.

The military junta, which seized power in a coup two years ago, derives its authority from the king. But the king’s heir apparent, Crown Prince Vajiralongkorn, seen by many as a jet-setting playboy, is not held in the same regard as his father.

King Bhumibol spent most of his final years in a hospital, ensconced in a special suite. His portrait hung in almost every shop, and as his health declined, billboards proclaimed “Long Live the King,” signaling widespread anxiety about a future without him. In response, he openly fretted about the people feeling so insecure.

Thais came to see this Buddhist king as a father figure wholly dedicated to their welfare, and as the embodiment of stability in a country where political leadership rose and fell through decades of military coups.

His death ends a reign of 70 years and 126 days, one that few monarchs have matched for longevity. Queen Elizabeth II, by comparison, has ruled Britain for more than 64 years, having surpassed Queen Victoria’s mark in 2015. With King Bhumibol’s death, she becomes the world’s longest-reigning monarch.

King Bhumibol (pronounced poo-me-pon) was an accidental monarch, thrust onto the throne at 18 by the violent death of his older brother in 1946. He fully embraced the role of national patriarch, upholding Thailand’s traditions of hierarchy, deference and loyalty.

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King Bhumibol Adulyadej, a Unifying Figure for Thais

King Bhumibol Adulyadej, a Unifying Figure for Thais

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Western stereotypes of his country irked him. He disdained the Broadway musical “The King and I,” with its roots in his grandfather’s court. And, like a stern father, he was quick to chastise his fellow Thais when he saw the need.

In the king’s own book “The Story of Tongdaeng” (2002), about a street dog he had adopted, the message — there was always a message in his writings — was that affluent Thais should stop buying expensive foreign breeds when there were so many local strays to save. The book was a Thai best-seller.

If he was a people’s king, Bhumibol was a quiet and somewhat aloof one. He was a man of sober, serious mien, often isolated in his palaces, protected by the most stringent of lèse-majesté laws, which effectively prevent almost any public discussion of the royal family.

But he had a worldly bent. Born in Cambridge, Mass., where his father was a student at Harvard, he was educated in Switzerland, spoke impeccable English and French, composed music, played jazz on the clarinet and saxophone, wrote, painted, took up photography and spent hours in a greenhouse at his Chitrlada Palace in Bangkok.

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The Life and Rule of Thailand's King

King Bhumibol Adulyadej of Thailand, the world's longest-reigning monarch who was highly revered by his nation, died at 88 after a prolonged period of illness.

By RICHARD C. PADDOCK and CAMILLA SCHICK on Publish Date October 13, 2016.
Photo by Adam Dean for The New York Times.
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Once he had returned from Europe, however, he stayed put. Never interested in a jet-set life, he stopped traveling abroad, saying there was too much to do at home. He was content to trudge through croplands in distant provinces in an open-neck shirt and sport coat, tending to the many development projects he encouraged and oversaw: milk-pasteurizing plants, dams that watered rice fields, factories that recycled sugar-cane stalks and water hyacinths into fuel, and countless others.

In a political crisis, Thais admired him for his shrewd sense of when to intervene — sometimes with only a gesture — to defuse it, even though he had only a limited constitutional role and no direct political power.

“We are fighting in our own house,” he scolded two warring politicians he had summoned to sit abjectly at his feet in 1992. “It is useless to live on burned ruins.”

Eleven years earlier, he had aborted a coup by simply inviting the besieged prime minister, Prem Tinsulanonda, to stay at a royal palace with the king and queen.

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Prince Bhumibol in 1935 with his older brother, Ananda, at right, who was king of what was then known as Siam.Credit
Associated Press

Thailand was transformed during his reign, moving from a mostly agricultural economy to a modern one of industry and commerce and a growing middle class. He presided over an expansion of democratic processes, though it was halting. He witnessed a dozen successful military coups and several attempted uprisings, and in his last years, his health failing, he appeared powerless to stem sometimes violent demonstrations, offering only vague appeals for unity and giving royal endorsement to two coups.

Meanwhile a strain of republicanism emerged as the country broke into two camps: on one side, the establishment, with the palace at its core; on the other, the disenfranchised, whose demand for a political voice threatened the traditional order.

In his birthday speech in 2005, he said the belief that the king can do no wrong was “very much an insult to the king.”

“Why is it that the king can do no wrong?” he asked. “This shows they do not regard the king as being a human. But the king can do wrong.”

Bhumibol Adulyadej was born in Cambridge on Dec. 5, 1927, the son of Prince Mahidol of Songkhla, a founder of modern medicine in Thailand; the prince was studying public health at Harvard at the time.

Bhumibol’s mother, Princess Sangwalya Chukramol, was a Thai nurse studying on a scholarship at Simmons College in Boston when she met the prince. Bhumibol had an older brother, Ananda, and a sister, Galyani Vadhana.

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King Bhumibol with Queen Elizabeth II of Britain, now the world’s longest-reigning monarch, in Bangkok in 1972.Credit
Associated Press

Bhumibol and his father were inheritors of the reformist tradition begun by King Mongkut in the 19th century and accelerated by his son King Chulalongkorn, Bhumibol’s grandfather.

Mongkut and Chulalongkorn were the king and prince in “Anna and the King of Siam,” Margaret Landon’s 1943 novel, which was based on the autobiographical writings of Anna Leonowens. The novel inspired the musical “The King and I” and its film adaptation.

Bhumibol was 2 when his father died, and his mother, to whom he was very close, took her children to Switzerland for schooling. Their family life was interrupted in 1935 when Thailand’s last absolute king, Prajadhipok, Prince Mahidol’s half brother, abdicated after a military coup. The crown passed to Prince Mahidol’s eldest son, Prince Ananda, then 10 years old.

King Ananda was barely into his 20s when, on June 9, 1946, he was found dead in his private chambers with a bullet through his head. Bhumibol was the last family member to have seen him alive, but he never spoke publicly about the death or about rumors that the young king, a gun collector, may have committed suicide or killed himself accidentally.

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Mourners after an announcement that King Bhumibol Adulyadej of Thailand had died, outside the Siriraj hospital in Bangkok on Thursday.Credit
Adam Dean for The New York Times

Bhumibol, though not originally in the line of succession, was anointed king. He soon returned to Switzerland for a few years and studied politics and history at the University of Lausanne.

While on a trip to Paris, he met Sirikit Kitiyakara, whose father, a Thai prince, was serving as a diplomat in Europe. They married in 1950, the year King Bhumibol was formally crowned Rama IX of the Chakri dynasty.

In an interview with The New York Times in 1988, the first he gave to a Western newspaper, King Bhumibol spoke with some bitterness of his early reign. He was repeatedly silenced by the military when he tried to assert himself, he said, and so decided to focus on what he could do best within his limited rights. That led to his concentration on industrial and agricultural development, an area in which the military could not challenge him without further undermining its increasingly shaky popular support.

King Bhumibol began systematically building a following across the Thai political spectrum, down to the village level. It was a strategy emulated in neighboring Cambodia by Norodom Sihanouk, another Asian monarch who held the devotion of a nation through years of turmoil.

David K. Wyatt, the author of the classic 1982 book “Thailand: A Short History,” credited King Bhumibol with turning the monarchy into Thailand’s strongest social and political institution.

Queen Sirikit, though often ill, apparently from depression or a nervous disorder, tried to keep up with her husband as he toured the country and visited the more than 1,200 development projects he fostered. She concentrated on reinvigorating Thai handicrafts.

She and the couple’s four children survive. The eldest is the daughter Ubol Ratana, who graduated from the Massachusetts Institute of Technology, married an American and lived mostly in California until separating from him and returning to the fold in Thailand in 2006. The youngest, Princess Chulabhorn, has a degree in organic chemistry and was married to a Thai commoner. It was she who broke a royal silence about the health of her mother in the mid-1980s by saying that Queen Sirikit, an insomniac, suffered from exhaustion.

The heir to the throne, Prince Vajiralongkorn, is the only royal son.

A daughter, Princess Maha Chakri Sirindhorn, never married and had devoted herself to studying the arts and helping her father with his many projects. She has for years been the most popular woman in Thailand, a quiet, personable foil to her brother.

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Toward the end of the king’s life, Prince Vajiralongkorn was moving to the center of public life. The military has recently sought to burnish the image of the prince, a partnership that may also have cemented the generals’ power.

King Bhumibol was nearing the end of a long day of visiting projects in eastern Thailand in the summer of 1988 when he and Princess Sirindhorn agreed to be interviewed by The Times in a Swiss-style chalet by a reservoir built by a donor in honor of the king. The subject turned to the legend of “The King and I,” which had been banned in Thailand as being disrespectful to the monarchy, and to the West’s image of the glittering life of a king of Siam, embodied in the musical most memorably by Yul Brynner.

“At first it was all this rubbish about the half brother of the moon and the sun and master of the tide and all that,” King Bhumibol said in his fluent English. He said he found it “irking” to have to live up to legends created by Western writers.

“They wanted to make a fairy tale to amuse people,” he said, “to amuse people more than to tell the truth.”

In reality, he said, his life revolved around his development projects.

He said he did not care how history would remember him.

“If they want to write about me in a good way,” he said, “they should write how I do things that are useful.”

Correction: October 13, 2016

An earlier version of this obituary referred incorrectly to Thailand’s government in 1946. The country was in a brief period of civilian government at the time; it was not under military control. (The military seized power in Thailand in 1947 and has ruled off and on ever since.)

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Seth Mydans and Thomas Fuller contributed reporting.

A version of this article appears in print on October 14, 2016, on Page B14 of the New York edition with the headline: Bhumibol Adulyadej, 88, People’s King of Thailand, Dies After 7-Decade Reign. Order Reprints|Today's Paper|Subscribe